Split start triaging from confirm activity.

Before the PackageInstallerActivity had two duties:
- Select if we needed to stage and then launch the stage activity
- Let the user confirm the installation

This change splits the first one out into the InstallStart activity.

Also: - Only show window animation in first activity, suppress otherwise
      - Corectly read and persist source/caller information in start and
	later just read it from the intent.

Test: Installed an app
Change-Id: Iea94ddca1162d8f90a8d0bf7d21974fee883b199